Single supply headphone driver/charge pump combination
A headphone driver amplifier operative from a single DC voltage supply, coupled directly to the headphone speakers without the need for DC coupling capacitors used for preventing DC reaching the headphones. An onboard power supply generates a negative voltage rail which powers the output amplifiers, allowing driver amplifier operation from both positive and negative rails. Since the amplifiers can be biased at ground potential (0 volts), no significant DC voltage exists across the speaker load and the need for DC coupling capacitors is eliminated.
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This invention relates generally to amplifier circuits and more particularly to headphone driver amplifier circuits operating from a single positive voltage supply.
BACKGROUND OF THE INVENTIONSystems and devices using headphones are ubiquitous in many fields of technology. The trend to miniaturize electronic devices has resulted in the need for smaller headphone devices.
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Each headphone may be represented by a resistive headphone load to be driven by the incoming signals. Typical value for the resistive load of a headphone speaker is in 16 to 32 Ω(ohm) range.
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As shown in PRIOR ART
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Therefore, it is desirable to provide a circuit headphone amplifier system that could operate from a single positive voltage supply, and which does not require the usual large DC coupling capacitors or need the physical isolation of the common return of the headphones.
SUMMARY OF THE INVENTIONThe system of the present invention, allows a headphone driver amplifier to operate from a single voltage supply, yet does not require the usual series coupling capacitors used for preventing DC current from reaching the headphones. An on-board power supply generates a negative voltage rail, which powers the output amplifiers, allowing driver amplifier operation from both positive and negative rails. In this way, the amplifier can be biased at ground (0 volts) potential, generating no significant DC voltage across the headphone load (the headphones speakers).
Briefly, one aspect of the present invention is embodied in a circuit enabling a headphone driver amplifier to operate from a single voltage supply comprising of an amplifier having an output coupled to a headphone, the amplifier having a first and a second power supply lead, the first power supply lead connected to a positive supply voltage, and a DC voltage to voltage converter having an output and a power source lead connected to the positive voltage supply, the output of the charge pump circuitry connected to the second power supply lead, and the charge pump generating an output voltage at the output that is substantially equal in magnitude to some quanta of the negative of the power supply voltage.
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Prior art headphone driver systems for portable devices operate off of a single power supply, requiring the biasing of the headphones at mid-range of the power supply in order to fully represent the incoming signal without the danger of any clipping. As a result, these prior art systems require DC blocking capacitors to be used in series with the amplifiers driving the headphones. The value and physical size of these DC coupling capacitors are prohibitively large and limit miniaturization highly desired in most systems.
One aspect of the present invention allows for a headphone driver/amplifier circuits to operate off of a single voltage supply, without requiring the usual series coupling capacitors necessary for preventing DC current from reaching the headphones An on-board power supply generates a negative voltage rail, which powers the output amplifiers, allowing driver amplifier operation from both positive and negative rails. In this way, the amplifier can be biased at ground (0 volts) potential, generating no significant DC voltage across the headphone load (the headphones speakers).
The term “charge pump ” refers to a type of DC voltage to voltage converter that uses capacitors and in an alternative embodiment inductors to store and transfer energy. One type of charge pump (also referred to as switched-capacitor converters) includes a switch/diode network that charges and discharges one or more capacitors.
Alternatively, in implementing the present invention, a DC voltage to voltage converter may be used that includes an inductor.
The charge pump circuitry of the present invention generates a negative voltage rail −VDD with respect to ground, powering the output amplifiers and allowing driver amplifier operation from both positive and negative rails. Providing a negative voltage rail with respect to ground allows for the headphone amplifiers to be biased at ground voltage, allowing for the incoming signals to be amplified without clipping. As shown in
Returning to
The headphone amplifier circuit 45 allows for the headphone 10 to be biased at zero volts, operating between VDD and −VDD which in turn allows for the leads 50 and 52 of the respective headphone amplifiers 46 and 48 to directly couple the headphone speakers 10 to the headphone amplifiers 46 and 48 without the need for any DC coupling capacitors in series.
In the simple capacitor based IC charge pump circuitry 66, the switch network 74 and 76 toggles between charge and discharge states. An oscillator (OSC) 72 controls the two switches (74 and 76) that alternately charge a flying capacitor (C1) from an input voltage supplied by the amplifier 68 and 70, and discharge the flying capacitor (C1) into an output capacitor (C2). The voltage thus produced across the output capacitor C2 may be output as the output voltage (VOUT). Typically, the oscillator 72, the switches 74 and 76, and still other controls are all commonly contained in a single integrated circuit (IC).
The simple capacitor based IC charge pump circuitry 66 is of the step-up type, and it operates by stacking the potential of the charge in the flying capacitor Cl onto the potential of the input, and then charging the output capacitor C2 with this. The optimal result of this is an output voltage VOUT which is double that of the input voltage.
Those skilled in the electronic arts will readily appreciate that switched connections to the flying capacitor can be changed to simply shift charge from the input to the output, rather than to stack it as above. One very common type of step-down charge pump operates in this way, but further includes an appreciable resistance in the charge path to the flying capacitor. The resistance intentionally introduces a delay in the charging of the flying capacitor, and appropriate control of the oscillator is then used to switch the charge before it is able to reach the full input voltage potential. This type of charge pump may accordingly transfer charge quanta having only one-half, two-thirds, etc. of the input voltage, and thereby produce an output voltage which is correspondingly lower than the input voltage. This type of step-down charge pump is probably overwhelmingly the most common today, but it is not the only type possible. Alternative circuit arrangements allow for the generation of an output voltage VOUT which is equal to some negative quanta of the input voltage.
The foregoing examples illustrate certain exemplary embodiments of the invention from which other embodiments, variations, and modifications will be apparent to those skilled in the art. The invention should therefore not be limited to the particular embodiments discussed above, but rather is defined by the following claims.
Claims
1. An amplifier circuitry for directly driving stereo headphones, said amplifier circuitry being driven by a single supply voltage VDD, said amplifier circuitry comprising:
- a first and a second amplifier, the first amplifier having an output directly coupled to a first headphone and the second amplifier having an output directly coupled to a second headphone, each of the first and second amplifier having a VDD power supply lead connected to a positive voltage supply VDD; and
- a charge pump circuitry output connected to a −VDD supply voltage of the first and second amplifier, wherein said charge pump circuitry output provides a voltage substantially equal in magnitude to the negative value of the VDD supply, said charge pump further having a power supply lead connected to the VDD supply voltage.
2. A direct drive charge pump enabled stereo headphone system comprising the following formed on a single integrated circuit;
- a single power input for providing, internal to said integrated circuit, a VDD supply voltage originating external to said integrated circuit;
- a charge pump coupled to said single power input and operable to provide, internal to said integrated circuit, a voltage substantially equal in magnitude to the negative value of said external VDD supply;
- a first headphone amplifier power by both said external VDD supply and said voltage substantially equal in magnitude to the negative value of said external VDD supply, said first headphone amplifier having a first audio input driven by a first audio signal provided external to said integrated circuit, and a first audio output suitable for directly driving a stereo headphone;
- a second headphone amplifier powered by both said external VDD supply and said voltage substantially equal in magnitude to the negative value of said external VDD supply, said second headphone amplifier having a second audio input driven by a second audio signal provided external to said integrated circuit, and a second audio output suitable for directly driving said stereo headphone; and whereby a single ground reference may be used for both said headphone system and said stereo headphone such that substantially no DC bias voltage exists across said first audio input and said second audio input with reference to said stereo headphone.
3. A direct drive charge pump enabled stereo headphone system as recited in claim 2, wherein said first audio input is an audio in for a right stereo headphone and said second audio input is an audio in for a left stereo headphone.
4. A direct drive charge pump enabled stereo headphone system as recited in claim 2, wherein said first audio input is coupled to an inverting terminal of said first headphone amplifier and said second audio input is coupled to an inverting terminal of said second headphone amplifier.
5. A direct drive charge pump enabled stereo headphone system as recited in claim 4, wherein a non-inverting terminal of said first headphone amplifier is coupled to a non-inverting terminal of said second headphone amplifier.
6. A direct drive charge pump enabled stereo headphone system as recited in claim 5, wherein said non-inverting terminal of said first headphone amplifier and said non-inverting terminal of said second headphone amplifier are coupled to ground.
7. A direct drive charge pump enabled stereo headphone system as recited in claim 2, further comprising:
- a first resistor coupled to said first audio input and an inverting terminal of said first headphone amplifier,
- a second resistor coupled to said first audio output and said inverting terminal of said first headphone amplifier,
- a third resistor coupled to said second audio input and an inverting terminal of said second headphone amplifier, and
- a fourth resistor coupled to said second audio output and said inverting terminal of said second headphone amplifier.
8. A direct drive charge pump enabled stereo headphone system as recited in claim 2, further comprising:
- a first capacitor having a first lead and a second lead, said first lead coupled to said charge pump and said second lead coupled to said charge pump, said first capacitor located off said integrated chip, and
- a second capacitor having a first lead and a second lead, said first lead coupled to said charge pump and said second lead couple to ground, said second capacitor located off said integrated chip.
9. A direct drive charge pump enabled stereo headphone system as recited in claim 2, further comprising a short circuit protection device.
10. A direct drive charge pump enabled stereo headphone system as recited in claim 2, further comprising a bias circuitry device.
11. A direct drive charge pump enabled stereo headphone system as recited in claim 2, wherein said first audio output is an audio out for a right stereo headphone and said second audio output is an audio out for a left stereo headphone.
12. A direct drive charge pump enabled stereo headphone system as recited in claim 2, further comprising a click/pop suppression device coupled to said first headphone amplifier and said second headphone amplifier.
13. A direct drive charge pump enabled stereo headphone system as recited in claim 2, further comprising a shutdown control device coupled to said charge pump.
14. A direct drive charge pump enabled stereo headphone system as recited in claim 12, further comprising a shutdown control device coupled to said charge pump and said click/pop suppression device.
15. A direct drive charge pump enabled stereo headphone system as recited in claim 2, further comprising:
- a first capacitor having a first lead and a second lead, said first lead coupled to a non-inverting terminal of said first headphone amplifier and said second lead coupled to said first audio input, said first capacitor located off said integrated chip, and
- a second capacitor having a first lead and a second lead, said first lead coupled to a non-inverting terminal of said second headphone amplifier and said second lead coupled to said second audio input, said second capacitor located off said integrated chip.
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Type: Grant
Filed: Jan 24, 2002
Date of Patent: Jun 13, 2006
Patent Publication Number: 20030138112
Assignee: Maxim Integrated Products, Inc. (Sunnyvale, CA)
Inventor: Tony Doy (Sunnyvale, CA)
Primary Examiner: Henry Choe
Attorney: Perkins Coie LLP
Application Number: 10/056,994
International Classification: H03F 3/04 (20060101);